A METHOD OF PRODUCING SUPERCONDUCTING TAPES WITH A HIGH CURRENT DENSITY

Abstract: A method of producing superconductors with a relatively high critical current density by a mechanical deformation provided by means of a tool mechanism comprising movable clamping jaws (2) and hinged brackets and eccentric arms connected to said clamping jaws, which are formed in such a way that the final product is of the same thickness everywhere. According to the invention the mechanical deformation is performed continuously or stepwise continuously by the clamping jaws (2) and the hinged brackets and eccentric arms connected thereto being formed such that during the deformation the same relative coordinate applies in the longitudinal direction, whereby the deformation in the longitudinal direction is neglectable. The resulting deformation method highly resembles a uni-axial pressing.

Description: Title: A method of producing superconducting tapes with a high current density- Technical Field The invention relates to a method of producing superconductors, especial- ly in form of tapes with a relatively high critical current density, by a mechanical deformation provided by means of a tool mechanism compris¬ ing movable clamping jaws and hinged brackets and eccentric arms con¬ nected to said clamping jaws, which are formed in such a way that the final product has the same thickness everywhere. Background Art It is known that high-temperature superconducting tapes can be produced by filling a mixture of metaloxide powders into a metal pipe whereafter the pipe is deformed by way of pulling into a round wire followed by an extru¬ sion, a rolling or a pressing into a flat tape. Then the tape is heat-treated in such a manner that the powder is sintered and form a coherent, super¬ conducting mass. These mechanical and thermal processes can be repeat¬ ed in order to improve the superconducting properties, including the critical current density. Metaloxide powders are for instance YBa 2 Cu 3 O 7 , Bi2. x Pb x Sr 2 Ca 2 Cu3θ , Tl2Ba 2 Ca 2 Cu3O y in polycrystalline form, in which they are slightly plastic and very brittle. The metal coatings can be made of Ag or Ag-alloys which unlike the above powders are very plastic and brittle. It is difficult to con¬ trol a mechanical deformation of composite articles made of such materials because the materials possess differing material liquidities. A ceramic, superconducting article must, however, possess predetermined structural properties in order to obtain a high critical current density. As few micro- cracks as possible must apply together with a high degree of texture with the superconducting Cu-O planes parallel to the current direction, and a high degree of uniformity with respect to the density, as well as a good electric interaction between the superconducting grains. It is important for obtaining these features that it is possible to control and optimize the forces having an effect on the article during the mechanical deformation. It has been demonstrated on small lengths of tape, viz. 2 to 8 cm, that the critical current density can be improved by 3 to 4 times provided the article after the first rolling and heat-treatment is subjected to one or more cycles involving a uniaxial force applied perpendicular to the current direc- tion and followed by a sintering. It is, however, difficult to obtain such a procedure by a continuous process, which is presumably necessary for allowing a use of the articles for the production of tape-shaped wires. During a rolling of a wire into a tape, the deforming forces can be divided into a pressing and a displacing force. During a rolling, these two forces are non-homogeneously distributed in the tape with the result that the superconducting ceramics are non-homogeneously compressed. In addi¬ tion, the displacing force causes microcracks transverse to the current direction in the plane of the tape, either during the rolling or during the following sintering where possible residual tensions are released. The deformation caused by the rolling results in a higher material liquidity in the longitudinal direction than in the transverse direction of the tape. Ceramic crystals already developed can thereby be broken during succeed¬ ing rollings with the unfortunate result that microcracks may arise in the transverse direction. A uniaxial pressing provides a very uniform power effect without notice¬ able displacing forces in the longitudinal direction of the wire. Such a pressing causes the material to be liquidized substantially only in the trans¬ verse direction. In addition, such a deformation reduces the formation of microcracks transverse to the current direction. A uniaxial pressing can, however, not be performed continuously. Brief Description of the Invention The object of the invention is to provide a method of continuously deform¬ ing an article substantially without deformation in the longitudinal direc- tion. A method of the above type is according to the invention characterised in that the mechanical deformation is performed continuously or stepwise continuously, and that the clamping jaws and the hinged brackets and eccentric arms connected thereto are formed such that during the defor- mation the same relative coordinate applies in the longitudinal direction, whereby the deformation in the longitudinal direction is neglectable. The resulting deformation method highly resembles a uniaxial pressing. A tool mechanism ensuring the above procedure can be a continuously driven multi-member mechanism, but which only subjects the article to a processing during a limited period of the process. The mechanism moves the clamping jaws in such a manner that only some of these clamping jaws are in contact with the article until a predetermined time and such that the pressing of the article is progressive, i.e. the processed length of the article is gradually increased. The quasi uniaxial pressing is obtained by the clamping jaws clamping on a portion of the article which has already been completely pressed while a portion not being completely pressed is subjected to the pressing. The resulting yielding of the article in its longi¬ tudinal direction is thereby minimized. Furthermore according to the invention the clamping jaws and the hinged brackets and eccentric arms connected thereto may be formed such that the power transmission to the clamping jaws is optimized by the angle between an eccentric arm and a hinged bracket connected thereto is close to 0° or 1 80° when the power transmission is maximal. Each arm is synchronously subjected to an input moment and an angular speed. In order to make the transmission of the torque as efficient as possible, the mechanism is formed in such a way that the angle of transmission of the torque is good on at least one of the arms as long as the article, viz. the wire, is subjected to a processing by the clamping jaws. The transmission of power can be further improved by the mechanism transmitting the power from the processing into frame in directions where said frame can be reinforced. In addition according to the invention the tool mechanism may be formed in such a way that the maximum rigidity is obtained, which is obtained by the elements between the clamping jaws and the frame being made as short as possible. The elements must, however, be able to provide the desired curve. Furthermore according to the invention the clamping jaws and the hinged brackets and eccentric arms connected thereto may be formed such that the largest possible length is processed per time unit. In practise a tool mechanism is chosen which provides a moving curve for the clamping jaws presenting the longest approximately linear segment when measured in the angular turning on the input side. Brief Description of the Drawings The invention is explained in greater detail below with reference to the accompanying drawings, in which Fig. 1 illustrates a method of producing a tape-shaped article in form of a superconducting tape, Fig. 2 illustrates a tool machine for continuously deforming tape-shaped articles and comprising two hinged bracket moving mechanisms, Fig. 3 illustrates a hinged bracket moving mechanism for a tool part of the tool machine, Fig. 4 is a perspective view of the tool part of Fig. 3, Fig. 5 is a side view of the entire tool machine, Fig. 6 is a sectional view of the tool machine of Fig. 5, Fig. 7 illustrates a round wire wound on a cylindrical coil holder, Fig. 8 illustrates the round wire of Fig. 7 subjected to a planisostatic press- ing, Fig. 9 illustrates a round wire wound into a flat coil structure, Fig. 1 0 illustrates the round wire of Fig. 9 subjected to a uniaxial pressing, and Fig. 1 1 illustrates a round wire wound into a flat coil structure and being subjected to a planisostatic pressing. Best Mode for Carrying Out the Invention Below a tool machine for the production of tape-shaped articles, such as superconducting tapes is described. A mechanical technology is dealt with where a wire composed of various materials is provided. The wire is subsequently subjected to a mechanical deformation which can be com¬ pared with a uniaxial pressing. The entire production process appears from Fig. 1 A powder of ceramic superconducting material is filled into a pipe of silver. The pipe is subjected to a pulling in order to reduce the diameter and then subjected to a sintering at approximately 830°C. The sintering is followed by a pressing or a rolling to form tapes. The production of superconducting tapes involves a number of factors implying that predetermined requirements must be presented to the mech¬ anical processing and consequently to the tool machine. The most import¬ ant requirements are based on the demand for - a desired texture, a uniform distribution of the power, a continuous production process, and a major supply of energy during the process. The first two requirements relate to the superconducting properties of the tape whereas the last two requirements relate to the necessary time and consequently to the production costs. The final superconducting tapes must have a texture not causing cracks in a direction perpendicular to the longitudinal direction of the tape. Accor¬ dingly, it is important that during the production the tape is not subjected to forces causing displacing tensions in the longitudinal direction of the tape. During the production of long tapes from a wire it is therefore important that all cross sections of the wire are subjected to the same force, and that this force is the same in all cross sections of the wire. The production process must be continuous or stepwise continuous in order to allow a production of long tapes and a reliable process with con¬ trollable parameters. Finally, a continuous production process results in a short production period. In order to reduce the production period and indeed in order to avoid a strong compression of the superconducting powder, it is important that relatively high amounts of energy can be supplied during the production process. Previously, the production of superconducting articles based on ceramic powder involved two processing procedures, viz. a pressing and a rolling. The production of tapes by way of pressing renders it possible to obtain uniform forces on all the cross sections of the wire. Furthermore, the forces act perpendicular to the longitudinal direction of the wire which improves the orientation in the powder. The pressing is, however, encum¬ bered with the drawback that the article is processed in one step, which means that the mechanical processing must be performed in one step. Such a procedure presents increased requirements to the tool machine and limits the applicability of the process. In addition, a pressing is a discon¬ tinuous process. Conventional rollers were previously used for the production of supercon- ducting tapes. During a rolling, the wires were subjected to a mechanical effect involving both a displacing tension and a pressing tension. The rolling results in a texture, viz. material properties depending on the direc¬ tion, where in particular the lattice orientation in the ceramic material is of major importance. The displacing tension causes the movement through the rollers. Attempts have been made at minimizing the displacing tension by means of various pulling rollers, whereby the rolling can be performed without noticeable displacing tensions. The article is, however, subjected to some displacing tensions by the pulling rollers, and the resulting texture has not been improved. The rolling process is continuous and can be used for the production of tapes of an arbitrary length. The rollers can be formed so as to supply relatively high amounts of energy to the wire. However, the displacing tension in the wire is correspondingly increased. Rollers are available for the production of thin articles and for the production of articles by means of a strong power effect. By the production of superconducting tapes by means of rollers, the radii of said rollers must be as large as possible in order to reduce the displacing forces. Large roller diameters limit, however, the rigidity and consequently the possibilities of producing thin wires or tapes. Attempts have been made by a method according to the invention at combining the advantages of rolling and pressing by means of particularly shaped tool parts in form of clamping jaws 2 simultaneously with avoiding some of the drawbacks. The moving pattern of the clamping jaws 2 allows a semicontinuous production process, a production of long lengths, a uniform distribution of the forces, a uniform distribution of the energy, and a high supply of power and energy. The wire-forming process is semicontinuous because only a limited portion of the wire is subjected to a processing at a time while in principle it is possible to produce extremely long tapes. A suitable choice of moving pattern and curve shape of the clamping jaws being in contact with the wire or the tape during the producing process renders it possible to achieve a uniform distribution of the forces in all the cross sections of the article. In addition, as all the cross sections of the article are processed in several steps, it is possible ' to obtain a uniform supply of energy over the entire length of the article. The processing of the wire can be compared with a rolling process by means of a roller of a very large diameter. When corresponding roller diameters are employed by a conventional rolling process it is, however, only possible to produce articles of relatively large thicknesses due to the fact that the resilient bending outwards of the rolling material exceeds the deformation necessary for achieving the yielding. The gripping of the wire by the tool parts is performed in the same manner as during a rolling process with the result that no wire cross sections are subjected to stronger displacing tensions than the neighbouring cross sec¬ tions. Although the clamping jaws 2 are not in constant contact with the wire, the drawbacks of the pressing have nevertheless been avoided. A semicontinuous production of wire is thus a processing allowing a high uniform supply of energy for long wires or the like. In order to obtain a mechanically rigid structure, the machine is designed with the shortest possible distance between the frame and the wire. A distance has been obtained which is less than 1 /50 in connection with rollers of a corresponding roller diameter. The supply of energy from the driving unit to the clamping jaws 2 must be such that the angles of transmission are advantageous in the period in which the tool parts subject the wire to a processing. Accordingly, a moving mechanism has been chosen which allow supplies of energy through several rotating driving shafts. In this manner it is possible to use toggle-joint-like members in the moving mechanism and thereby to obtain advantageous angles of transmission at the same time as strong forces can be transmitted. The moving mechanism comprises nine members and presents three degrees of freedom. Accordingly, the mechanism must comprise three synchronous driving shafts for the supply of energy in order to be deter¬ mined. The tool machine comprises two separate moving mechanisms with clamping jaws 2 operating agains each other, but which are supplied with energy from the same driving unit and thereby moved synchronously. Once the moving pattern has been calculated, the final curve shape for the clamping jaws has been determined. The simulation of the moving mechanism involves calculations which can only be performed by means of a computer. Several programs exist for this purpose, such as for instance CADME. The fundamental structure of the mechanism in the program is initially defined by means of this program by combining standard mechanisms from a library encoded in the program. Subsequently, the positions of the driving shafts, the frame etc. are defined. CADME can both analyze and synthesize the mechanisms. Here it is only used for analyzing. An analysis can either be in form of a simula¬ tion on a screen or in form of a transfer of data to a file. The use of the program involves 1 ) a cinematic simulation of various quantitative mechanisms so as to determine a combination of parameters providing optimum properties in the following areas a) track curves for points of interest on the surface of the clamp¬ ing jaw, b) distribution of forces and moments, c) consideration of the structure desired in relation to the pinions and moving mechanisms from a structural point of view, 2) calculation of coordinates during the running of the program for points of interest defined by the user. These coordinates are encoded in a text file and used for determining the curve shape of each clamp¬ ing jaw. Modifiable parameters are: a) The coordinates of the centres of rotation A, B, and C in a frame coordinate system, cf. Fig. 3, β) the lengths of the eccentric arms b 1 # b 2 , and b 3 , y) the phase displacements of the eccentric arms a^ , σ 2 , and σ 3 , δ) the points D, E, and F of the clamping jaws in the frame coordinate system. It is determined that the securing points D, E, and F must have the same X-coordinate to the time t = 0. The lengths of the hinged brackets b 4 , b 5 , and b 6 connected to the clamping jaws 2 in the points D, E, and F are then geometrically determined. Ad 1 a) When the clamping jaws 2 are to advance the wire 4, the points on the surfaces of the clamping jaws must present track curves in form of ellipses with substantially horizontal major axes, cf. Fig. 3. The appearance of the track curves reveals the horizontal and vertical movement Δx and Δy of the clamping jaws 2. Δx is set to 3 mm and Δy is set to 0.5 mm. Ad 1 b) It is desired that a strong force exerted on the clamping jaws 2 only necessitates a small torque on each driving shaft. The latter is obtained by dimensioning the tool machine in such a way that the most loaded driving shafts comprises the smallest eccentric arms and vice versa, and such .that the angle φλ , φ2, and 03 between an eccentric arm and a hinged bracket is close to 0° or 1 80° when the maximum force applies. Ad 1 c) In order to make room for shafts and possible pinions of suffiz- ing dimensions, the most critical distances must be maximal without changing the cinematic properties. The critical dist- ances are BE, CF, and the distance from the arm b 5 to the centre of rotation B. A specific embodiment provided by means of the program CADME results in the following coordinates for the centre of rotation to the time t = 0: D = (45,25) E = (80,25) F = ( 140,25) A = (0,0) B = (95,-1 5) c = ( 1 25,-75) The lengths of the eccentric arms are b-, * = 3.5 mm b 2 = 0.5 mm b 3 = 1 .0 mm The phase displacements of the eccentric arms are σ, = 280 ( a 2 = 0° 3 = 0° A tool machine is thus provided according to the invention, which can reduce the thickness of a wire 4 by a semicontinuous pressing, which means that a superconducting wire can be formed continuously by a processing corresponding to a rolling with a roller diameter of approximate¬ ly 3 m. Such a roller diameter renders it possible to compare the grain structure of the wire and the associated superconducting properties with the properties of pressed wires. The most important properties of the tool machine are as follows: it can simulate a rolling process by means of rollers of a diameter of approximately 3 m, it can press with a force of up to 100 kN, it can form wires of a thickness of up to 8 mm and a width of up to 1 5 mm, it can operate with broad clamping jaws in such a manner that wire widths of up to 35 mm can be processed, the distance between the clamping jaws can be adjusted with an accuracy of less than 5 μm, - up to 1 20 pressings per minute can be performed. The wire is advanced approximately 4 mm per pressing, which means that up to 14 m of wire per minute can be produced in connection with 1 20 pressings per minute. The tool machine is particularly rigid, and a good reproducibility is obtained. The fact that the current-related properties can be improved by a uniaxial pressing can be utilized for producing particular structures, such as coil structures. Figs. 7 to 1 1 illustrate examples thereof. A metal pipe is filled with raw powder and pulled into a long, thin, circular wire. Then the wire is formed into a coil-like structure which is subjected to a mechanical deformation followed by a heat-treatment. Fig. 7 shows a round wire wound on a cylindrical coil holder along a helix into a cylindrical coil structure. The coil structure is then subjected to a planisostatic pressing, cf. Fig. 8. The resulting magnetic field of the sole¬ noid is perpendicular to the pressing direction. Fig. 9 illustrates a round wire wound into a flat, almost helical coil struc¬ ture. Then a uniaxial pressing is exerted on the entire structure, cf. Fig. 1 0. Thereby the magnetic field of the coil extends parallel to the pressing direction. Fig. 1 1 illustrates a thin wire wound into a flat coil structure, and which instead is subjected to a planisostatic pressing so as to deform the coil. The resulting magnetic field of the coil is perpendicular to the pressing direction.

Claims: Claims. 1. A method of producing superconductors, especially in form.of tapes with a relatively high critical current density, by a mechanical deformation provided by means of a tool mechanism comprising movable clamping jaws and hinged brackets and eccentric arms connected to said clamping jaws, which are formed in such a way that the final product has the same thickness everywhere, c h a r a c t e r i s e d in that the mechanical deformation is performed continuously or stepwise continuously, and that the clamping jaws (2) and the hinged brackets and eccentric arms con- nected thereto are formed such that during the deformation the same relative coordinate applies in the longitudinal direction, whereby the defor¬ mation in the longitudinal direction is neglectable. 2. A method as claimed in claim 1, ch a racte ri sed in that the clamping jaws (2) and the hinged brackets and eccentric arms connected thereto are formed such that the power transmission to the clamping jaws (2) is optimized by the angle between an eccentric arm and a hinged bracket connected thereto is close to 0° or 180° when the power trans¬ mission is maximal. 3. A method as claimed in claim 1, ch a racterised in that the elements between the clamping jaws (2) and the frame are made as short as possible. 4. A method as claimed in one or more of the preceding claims, ch ar¬ acterised in that the clamping jaws (2) and the hinged brackets and eccentric arms connected thereto are formed such that the largest possible length is processed per time unit. 5. A method of producing a superconductor wound along a helix, op¬ tionally on a substantially cylindrical coil structure, ch a racte ri sed in that the helical superconductor is subjected to a planisostatic pressing (Figs.7 and 8). 6. A method as claimed in claim 5 of producing a superconductor wound along a flat helix, c h a r a c t e r i s e d in that the helical coil structure is subjected to a planisostatic pressing (Fig.11). 7. A method as claimed in claim 6, c h a r a cte ri s ed in that the helical coil structure is instead subjected to a uniaxial pressing (Figs.9 and 10).

Inventors: BROK KRISTIAN, DALUM LEIF, FRELTOFT TORSTEN, LI QI, BROK, KRISTIAN, DALUM, LEIF, FRELTOFT, TORSTEN, LI, QI
Applicants: BROK KRISTIAN, DALUM LEIF, FRELTOFT TORSTEN, LI QI, NKT RES CENTER AS, BROK, KRISTIAN, DALUM, LEIF, FRELTOFT, TORSTEN, LI, QI, NKT RESEARCH CENTER A/S
Application year: 1994
Main classification: H01L39
IPC codes: H01B12, H01B13, H01F6, H01L39